The mini-benchmark.sh script failed to run in the latest Fedora
distributions in GitLab CI. Executing the benchmark inside a Docker
container had failed because the check for `perf` was done in a way that
caused the benchmark to exit because of the `set -e` option. Test and
skip `perf` to allowing the remaining benchmark activities to proceed.

This check was added in acb6684 but inadvertantly reverted in 42a1f94.

Logic was corrected to only run perf when the flag is enabled, and to
prevent perf stat and perf record from being simultaneously enabled.

Set -ex is also added to enable easier identification of mini-benchmark
issues in the future.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.